The present invention relates to an electronic device having an anodic bonding structure consisting of a conductor or semiconductor and glass; and, more particularly, the invention relates to a micro-machine technique, including the use of micro-sensors and micro-pumps, and to an optical component.
Since anodic bonding can be employed to directly bond a semiconductor, such as Si, and glass, it is used mainly in the field of MEMS (Micro Electro Mechanical Systems) for the manufacture of micro-mechanical components by fabrication of Si. Since the principle of anodic bonding is reported in a significant amount of literature, such as Japanese Patent Laid-open No.10-259039, only an outline thereof will be described here.
When glass and Si are brought into contact with each other, and a DC voltage is applied using glass as a cathode and Si as an anode along with heating, positive cations contained in the glass are compulsorily diffused to the cathode to form a cation depletion layer near the bonding boundary with the Si. As a result, the cation depletion layer is rendered relatively anion rich where negative charges are accumulated, while positive charges are accumulated on the side of the Si with the bonding boundary being disposed therebetween, so that a large electrostatic attraction force is generated between the glass and the Si, thereby to cause bonding. Further, it has been known that not only the electrostatic attraction, but also a chemical reaction at a boundary between the Si and the glass exert a significant effect on the bonding force. This effect is described also in Japanese Patent Laid-open No. 10-259039.
Typical examples in which anodic bonding is actually applied include components of various kinds of sensors, such as pressure sensors, acceleration sensors and angular velocity sensors, or micro-pumps, typically represented by the inkjet nozzles of inkjet printers. These components are manufactured at first by applying anisotropic etching to the Si, and then by anodic bonding the same to a separate glass layer. The anodic bonding technique has been adopted for the products described above, since anodic bonding can directly bond Si and glass so as to enable extremely sensitive detection of a change in external pressure, etc.
The concepts involved will be explained by way of example with reference to a pressure sensor. In a pressure sensor, a recess is formed at a portion of a Si layer by anisotropic etching. The bottom portion of the recess of the Si layer is fabricated so as to be thin to such an extent that it is capable of being distorted by an external pressure. At this point, a resistor, such as a strain gauge, is formed at the thin bottom portion of the recess of the Si layer, and then the Si layer and a glass layer are anodically bonded to each other. A cavity having a volume corresponding to that of the recess formed by anisotropic etching is defined between the glass layer and the Si layer at the bottom of the recess, so that the thin Si layer will be distorted at the bottom of the recess in response to a change in the external pressure. Thus, the thin Si layer is distorted at the bottom of the recess in response to a change in the external pressure, by which the resistivity of the strain gauge changes, so that a change of pressure can be taken out instantaneously as an electrical signal.
Direct bonding of glass and Si has an advantage in that the change of external pressure, if it occurs, is directly transmitted to the Si layer. On the other hand, in a case where Si and glass are bonded together by using a bonding agent having a low elasticity, the deformation due to the change of external pressure, if any, is absorbed in the bonding agent and the deformation of Si per se is small, or the signals derived therefrom become instable. Further, since the devices described above are so extremely small that they are usually difficult to handle individually, they are fabricated and bonded while still in the wafer state. Wafers can be positioned and bonded with high accuracy, and anodic bonding is suitable to the fabrication of such products also in view of the fabrication processes. These are some of the reasons why anodic bonding has been used generally in the fabrication of various kinds of sensor components.
However, since anodic bonding employs a technique in which the wafers are bonded to each other in a solid state, if dust is present between the Si and glass layers, or the wafers include undulations, many voids are formed at the bonding boundary, which can lead to a bonding failure. In a case where the quantity of dust or the size of the wafer undulations is small, since the glass itself undergoes some deformation by heating it to or above a softening temperature, the occurrence of voids at the boundary may possibly be suppressed somewhat. However, the deformation of glass is usually small, and so dust or wafer undulations still tend to deteriorate the quality of the bonding.
In order to solve such a problem, Japanese Patent Application No. 10-259039 discloses a technique of incorporating a metal layer that forms eutectics with a conductor or semiconductor at a temperature lower than the softening point of a glass layer, whereby the adhesion between the glass and Si layers is improved by utilizing this technique. This document discloses a technique in which Au metallization is applied specifically to a surface of the Si layer, which is heated to or above 363° C. as an eutectic temperature of Au—Si to form molten eutectics, thereby closely bonding the boundaries, irrespective of the presence of dust or undulations on the boundaries. What is important in this technique is to liquefy the surface of the Si layer upon bonding, thereby absorbing the wafer undulations or dust into the liquid and increasing the adhesion between the wafers. That is, this technique inevitably requires liquid to be formed at the bonding boundaries.